It is known that conventional types of speed drives cannot meet the requirements of efficiency, exactness and cost because of their high installation cost, smaller torque at slow speed, high maintenance cost and high energy consumption. Through pulse width modulated (PWM) type of inverters can meet the aforementioned requirements, it has been found that the use of PWM inverters causes premature failure of enamelled wires because of the inverters' high peak voltage values, pulsed voltage surges and harmonics, boost up and down, and high switching frequencies. Specifically, pulsed voltage surges arise within a very short time, measured in microseconds, which causes the temperature to suddenly increase (e.g. the effect of pulse voltage surges on temperature is more greater that of cornea discharge). The sudden increase of temperature causes thermal-oxidation decomposition of the insulation coating layers on enamelled wires and shortens the life of the wires.
U.S. Pat. No. 5,654,095 discloses a pulsed voltage surge resistant enamelled wire which can withstand voltage surges approaching 3000 volts and is resistant to high temperatures up to 300.degree. C., where the rate of voltage increase greater than 100 kV/.mu.sec and the frequency is less than 20 kHZ. The enamelled wire of U.S. Pat. No. 5,654,095 is characterized by the addition of metal oxide particles having a particle size of from 0.05 to 1 micron to the shield layer of enamelled wire to provide the desired pulse voltage surge resistant. According to U.S. Pat. No. 5,654,095, metal oxides which can effectively resist pulse voltage surges and increase the lifetime of enamelled wires include titanium dioxide, alumina, silica, zirconium oxide, zinc oxide, iron oxide and various naturally occurring clays such as those listed in column 4, lines 57-59. Though the examples of U.S. Pat. No. 5,654,095 disclose that the metal oxide as Al.sub.2 O.sub.3, they are totally silent on the structure of that Al.sub.2 O.sub.3.
There are two major structural types of Al.sub.2 O.sub.3 --.alpha.-form and .gamma.-form. .alpha.-form is a trigonal (R-3CH) structure wherein the lattice constants a=b=4.8 .ANG. and c=13.0 .ANG., the lattice angles .alpha.=.beta.=90.degree. and .gamma.=120.degree.. .gamma.-form is a cubic (Fd-3mS) structure wherein the lattice constants a=b=c=7.9 .ANG. and the lattice angles .alpha.=.beta.=.gamma.=90.degree.. The structure of .alpha.-form Al.sub.2 O.sub.3 is more compact than that of .gamma.-form. In other words, the structure of .gamma.-form Al.sub.2 O.sub.3 is closer to an amorphous phase and is significantly different from that of a .alpha.-form.
It has been found that, in the shield layer(s) of an enamelled wire, the use of both .alpha.-form Al.sub.2 O.sub.3 particles and .gamma.-form Al.sub.2 O.sub.3 particles can provide pulse voltage surge resistance that is much better than that provided by .alpha.-form Al.sub.2 O.sub.3 particles or .gamma.-form Al.sub.2 O.sub.3 particles alone.